The invention relates to a process for wetting at least one surface of an electrolyte, which is in particular a proton-conducting electrolyte membrane, in a fuel cell, in particular a low-temperature fuel cell, and to a fuel cell.
As in other galvanic elements, in fuel cells the bond energy which is liberated, for example, during the chemical bonding of hydrogen (H2) and oxygen (O2) is converted into electrical energy and heat. There is a fundamental distinction drawn between low-temperature fuel cells (up to approx. 200xc2x0 C.) and high-temperature fuel cells (approximately 600 to 1100xc2x0 C.). Between these two classes are the so-called molten carbonate fuel cells (MCFC), which have a working temperature of approximately 200 to 600xc2x0 C. and have a liquid electrolyte disposed in a matrix.
High-temperature fuel cells, such as solid oxide fuel cells (SOFC) contain, for example, a solid electrolyte made from zirconia which is ion-conducting at a working temperature of 850 to 1050xc2x0 C. They are operated primarily in stationary installations and functioning as decentralized power supplies.
Low-temperature fuel cells in combination with an electric motor could form an alternative to conventional internal combustion engines, in particular in vehicles and railroad systems.
In known electric vehicles, the electrical energy is first generated in a power plant and then temporarily stored on board the vehicle in a battery. High costs, considerable weight, limited service life and long charging times for these batteries represent problems that have not been satisfactorily solved.
Therefore, concepts which do not require temporary storage, i.e. which generate power onboard and according to demand, therefore appear particularly promising; in particular, this is true of the concept of fuel cells which have a proton-conducting membrane electrolyte, known as a proton exchange membrane fuel cell (PEMFC). The gaseous fuel, in particular gaseous hydrogen and gaseous oxygen, does not have to be burnt, but rather is directly converted into electrical energy and steam in a so-called cold reaction. The electrolyte in the PEM fuel cell separates the two gases from one another and prevents a so-called hot reaction. An electrochemical process at the electrolyte only allows protons, i.e. positively charged hydrogen ions (H+), to pass through. The electrons of the hydrogen atoms are separated out as the hydrogen passes through the electrolyte and are retained, while the hydrogen ions react with the oxygen particles on the other side. On account of the excess of electrons on the hydrogen side and the lack of electrons on the oxygen side of the electrolyte, there is a difference in potential across the adjacent electrodes, so that when the electrodes are electrically connected via an external circuit which includes a consumer, an electric current flows from the anode to the cathode. In addition to the electrical energy, heat and water are formed as reaction products.
In the case of the low-temperature fuel cells, in particular the PEM fuel cells, the surfaces of the electrolyte and/or the electrolyte-side surfaces of the adjacent electrodes need to be kept moist in order to promote the reaction and achieve high levels of efficiency. For this purpose, it is known, for example, from German Patent DE 43 18 818 C2 for the fuel cell to be operated with a humidified gas which, however, has to be compressed beforehand at a relatively high cost in order subsequently to be miscible with a fluid, in particular with water.
It is accordingly an object of the invention to provide a process for wetting at least one of the surfaces of an electrolyte in a fuel cell which overcomes the above-mentioned disadvantages of the prior art methods and devices of this general type.
With the foregoing and other objects in view there is provided, in accordance with the invention, a wetting process. The process includes the step of providing a fuel cell having an electrolyte with at least one surface and at least one channel body with at least one semi-permeable membrane disposed above the electrolyte. A fluid containing a wetting agent is guided in the channel body of the fuel cell, it being possible for at least some of the wetting agent to pass through the semi-permeable membrane of the channel body to reach the electrolyte. The amount of the fluid to be supplied to the electrolyte is metered in dependence on a type of the fuel cell used and on parameters that can be adapted to a particular fuel cell configuration.
The invention is based on the object of providing a process for wetting at least one of the surfaces of an electrolyte, by which it is possible to ensure that the electrolyte is adequately wetted. A further object of the invention is to provide a fuel cell in which the outlay on an apparatus for wetting at least one surface of the electrolyte is relatively low.
The process according to the invention for wetting at least one surface of an electrolyte, which is in particular a proton-conducting electrolyte membrane, in a fuel cell is distinguished by the fact that the fluid which contains the wetting agent is provided in at least one channel body. At least some of the wetting agent can pass through at least one semi-permeable membrane of the channel body to reach the electrolyte. This process results in a safe, reliable wetting of at least one surface of the electrolyte.
The wetting agent is preferably water.
At least one of the surfaces of the electrolyte is at least partially provided, by the wetting agent guided in the channel body, with a layer of water. Supplying water in this way advantageously enables at least one of the surfaces of the electrolyte to be permanently and continuously provided with a layer of water, the reactions taking place on the electrolyte, in particular the ionization of the hydrogen atoms, being promoted by the layer of water. Preferably, at least that surface of the electrolyte which is on the fuel gas side is wetted, since the demand for wetting agent, on account of water being a reaction product, is lower on the surface on the reaction gas side.
According to the invention, in addition to the wetting agent the fluid preferably contains at least one carrier medium, the carrier medium used preferably being air. A fluid of this type can then advantageously be fed to the electrolyte through the porous electrodes.
In accordance with an added feature of the invention, the electrolyte is a proton-conducting electrolyte membrane.
In accordance with an additional feature of the invention, the fuel cell is a low-temperature fuel cell.
In accordance with another feature of the invention, there is the step of using temperature as one of the parameters.
With the foregoing and other objects in view there is provided, in accordance with the invention, a low-temperature fuel cell. The fuel cell contains two porous electrodes of different polarities each having a gas-side surface and an electrolyte-side surface. An electrolyte is disposed between the electrodes on the electrolyte-side surface of each of the electrodes. The electrodes each have surfaces including a surface on a fuel gas side and a surface on a reaction gas side. At least one channel body is configured such that at least one of the surfaces of the electrolyte is at least partially wettable by a fluid guided in the channel body and contains a wetting agent. The channel body has at least one semi-permeable membrane through which the wetting agent can pass to reach the electrolyte, and an amount of the wetting agent which is to be supplied to the electrolyte can be metered in dependence on a type of the fuel cell used and on parameters which can be adapted to a particular fuel cell configuration.
In accordance with a further feature of the invention, the channel body is disposed at least partially in an approximately meandering form.
According to the invention, the channel body is preferably disposed at least partially on the gas-side surface of at least one electrode and/or integrated at least partially in at least one electrode. The channel body may be formed or disposed at least partially as a reservoir in the form of a blind hole. It is also possible to provide a plurality of channel bodies that run substantially parallel to one another. In addition, the channel bodies are disposed substantially parallel to the surfaces of the electrode. The various configurations advantageously allow a simple, in particular permanent and continuous, supply of a wetting agent to at least one surface of the electrolyte irrespective of the configuration of the fuel cells, for example above one another in so-called stacks or in strip form next to one another.
According to the invention, the channel body preferably has at least one semi-permeable membrane that is preferably disposed on the electrolyte side in the channel body. The wetting agent can advantageously pass through the semi-permeable membrane to reach the electrolyte.
In its most simple configuration, the channel body is at least partially a plastic molding. Depending on what is expedient, the channel body may, however, also be at least partially formed by the electrode itself, which is advantageous in particular in terms of manufacturing technology.
According to the invention, the fuel cell is preferably cooled by the fluid and/or the channel body. For this purpose, the channel body is at least partially configured as a heat sink, preferably in ribbed form. Alternatively, or in addition, the channel body is connected to a cooling device, which is preferably configured as a cooling circuit. The cooling of the fuel cells is important in particular in the case of fuel gases and/or oxidation gases that are supplied at high pressure.
Preferably, according to the invention, the amount of wetting agent which is to be supplied to the electrolyte can be metered as a function of the type of fuel cell used and in dependence on parameters which can be adapted to the particular fuel cell configuration, in particular as a function of temperature.
The advantages achieved with the invention relate in particular to the fact that metering is possible via an adjustment in the pressure or via the concentration of the wetting agent in the fluid, in particular if the fluid additionally contains a carrier medium.
Other features which are considered as characteristic for the invention are set forth in the appended claims.
Although the invention is illustrated and described herein as embodied in a process for wetting at least one of the surfaces of an electrolyte in a fuel cell, it is nevertheless not intended to be limited to the details shown, since various modifications and structural changes may be made therein without departing from the spirit of the invention and within the scope and range of equivalents of the claims.
The construction and method of operation of the invention, however, together with additional objects and advantages thereof will be best understood from the following description of specific embodiments when read in connection with the accompanying drawings.